1. Field of the Invention
The present invention relates to a method of evaluating the performance of an optical sensor installed in an ink jet printing apparatus.
2. Description of the Related Arts
In recent years, various sensors for detecting and measuring printing conditions of an ink jet printing apparatus depending on the necessity are installed in the apparatus to enhance its image quality, precision for printing operation, and user friendliness. Examples of these sensors include: a sensor for detecting an end position and width (or size) of a printing medium; a sensor for detecting the presence or absence of a printing medium as well as the thickness of a printing medium; and a sensor for measuring the density and the like of a predetermined pattern printed on a printing medium.
The detection of an end position and width of a printing medium is effective for printing an image at an exact location on the printing medium. Particularly in a case of “margin-less printing,” in which an image is printed on a printing medium with no margin, it is desirable that the printing apparatus minimize the amount of ink ejected to the outside of the printing medium while preventing a margin from appearing due to shift of printing position. Even in this case, if the printing apparatus is capable of detecting an end position and width of the printing medium precisely, it is possible to control the printing position so as to print the image on the exact location on the printing medium. Thus, the printing apparatus is capable of achieving a highly precise “margin-less printing.”
In general, a reflection optical sensor configured of one light-emitting element and one light-receiving element is used for a sensor for detecting an end position of a printing medium. In such optical sensor, light radiated from the light-emitting element is reflected at a printing medium, and the light-receiving element receives the reflected light. Incidentally, an output from the light-receiving element changes depending on the intensity of the reflected light. While the radiated light is reflected at an end portion of the printing medium, the density of the reflected light is lower, and the output from the light-receiving element is accordingly lower. With this taken into consideration, if a threshold value of output for detecting an end portion is beforehand determined, it is possible to detect the end portion through comparison between a result of the output from the light-receiving element and its corresponding position. Carrying out this type of detecting operation at the two side ends of the printing medium enables the width of the printing medium to be calculated as well.
The detection of the thickness of the printing medium is effective for keeping a distance between the ink jet printing head and the printing medium (a distance to the paper) optimal. In a case where a sensor for detecting the thickness of a printing medium detects the thicknesses of printing media to find a thinner printing medium, the printing apparatus makes the printing head come closer to the printing medium, while in the case where a thicker printing medium is found, the printing apparatus makes the printing head go away from the printing medium, by adjustment. Accordingly, it becomes possible for the printing apparatus to keep the distance between the printing head and each printing medium constant even when the printing medium in a variety of thicknesses is loaded thereon. By keeping the distance to the paper constant, it is possible to control, with high precision, landing positions of ink droplets ejected from the printing head, and to administer the amount of mist generated in conjunction with the ejection of ink droplets.
In general, a reflection optical sensor which is termed as a range sensor is used for detecting the thickness of a printing medium. The range sensor has its optical system configured of a light-projecting element such as a LED (light emitting diode) or a laser and a light-receiving element such as a PSD (position sensitive detector) or a line sensor. Light radiated from the light-projecting element is reflected at a measuring object, and the light-receiving element receives the reflected light. The center of power of the reflected light received by the light-receiving element (or the position at which the light-receiving element receives the strongest power of the reflected light) changes as the distance between the sensor and the measuring object changes. For this reason, once the light-receiving element detects the position of the center of power of the reflected light, it is possible to calculate the distance between the sensor and the measuring object by triangulation method or the like.
The measurement of the density and the like of a predetermined pattern printed on a printing medium is effective for adjusting the density of an output image, and for controlling landing positions of the ink droplets. In the case of the ink jet printing apparatus, when the amount of ink droplets ejected from the printing head (the ink ejection volume) changes, or is different from one ink color to another, the density and hue of an outputted image become unstable. With this taken into consideration, multiple output conversion tables are beforehand set up in order that an output signal corresponding to an input signal can be changed depending on an ink ejection volume so as to cause a relationship between the input signal and the density of the output image to remain stable independent of the ink ejection volume. Then, for each of multiple ink colors, a predetermined gradation pattern printed in a single-color ink is printed and the density of each pattern is detected with its density sensor. Thereby, it is possible to set up an appropriate output table for each ink color depending on given detection values.
A reflection optical sensor of a type including light-emitting elements such as LEDs having red, green and blue light-emitting wavelengths as well as light-receiving elements sensitive to these wave ranges is often used for the sensor for detecting densities of the multiple color patterns. Used is another reflection optical sensor of a type including a light-emitting element such as a white LED for emitting light having red, green and blue wavelengths, three filters for allowing the red, green and blue wavelengths out of the reflected light to be transmitted through the three filters, as well as light-receiving elements for receiving the transmitted light rays. Once light suitable for a light-absorbing characteristic of a single-color ink with which a gradation pattern is printed on a printing medium is radiated on the gradation pattern, the amount of reflected light corresponding to the radiated light changes depending on the density of the pattern. By detecting the amount of this reflected light with the light-emitting elements, the density of each pattern is capable of being obtained. When a detected density is higher than the reference density, such an output conversion table is selected in which an output signal corresponding to the gradation of the pattern (or the input signal) is set to be lower than its standard level. Meanwhile, when the detected density is lower than the reference density, another output conversion table is selected in which an output signal corresponding to the gradation of the pattern (or the input signal) is set to be higher than its standard level. The foregoing density detection method is disclosed in, for example, Japanese Patent Application laid-open No. Hei05-346626.
As is often the case with the foregoing various types of optical sensors, each optical sensor is attached to a carriage, which scans with a printing head for ejecting inks being mounted on the carriage, and is positioned by the carriage at a location of an object printing medium in a detection. However, such a configuration causes the optical sensor to be exposed to an ink mist which occurs in conjunction with the ejection of inks from the printing head, and thereby to be gradually smeared as the number of printings increases. To put it concretely, the mist means fine ink droplets which are generated in conjunction with the ejecting operation of the printing head and due to rebound of ink droplets from the printing medium, and which float in the air between the printing head and the printing medium because of their slow velocities. When a printing operation is carried out with such a generation of mist having occurred, the mist floats in conjunction with the movement of the carriage, and part of the mist adheres to surfaces of the elements of the optical sensor. Once the mist adheres to the light-emitting elements and the light-receiving elements, the light is blocked by the mist. Then, the amount of light emitted from each light-emitting element and received by each light-receiving element accordingly reduces, resulting in decrease in the performance of the sensor itself. As a result, the various controls fail to be effectively carried out, which controls are to be made on the basis of a result of detection by each sensor. For this reason, in the case of the ink jet printing apparatus having the foregoing configuration, it is desirable that the function of the optical sensor should be recovered by providing the optical sensor with a maintenance service, adjustment or replacement with a new one, at an appropriate time. In this case, it is required that the timing for the function of the optical sensor to be recovered should be evaluated exactly so as to avoid an unnecessary maintenance service and an unnecessary replacement with a new optical sensor.
For example, Japanese Patent Application laid-open No. 2004-291601 discloses a method in which a mode for checking on output conditions of the sensor is beforehand provided, and in which the amount of light (or power) outputted from the sensor is adjusted when the value representing an output from the sensor goes under a predetermined threshold value. Further disclosed is a method in which, when the output value does not exceed the predetermined value regardless of the adjustment of the amount of light, a user is informed of the necessity for replacing a protective member attached to the sensor with a new one. This method makes it possible to prevent, by recovering the function of the optical sensor depending on the necessity, or by informing a user of the life of the optical sensor and the timing for the protective member to be replaced with a new one, detection from being made under a condition where the sensor does not work well, even while the performance of the optical sensor gradually decreases as the number of printings increases.
However, like in the sensor for detecting densities of each color pattern, in the optical sensor including the light-emitting elements having the red, green and blue light-emitting wavelengths as well as the light-receiving elements sensitive to those wave ranges, how the performance decrease is different depends on the color of the ink mist adhering to the light-emitting elements and the light-receiving elements of the optical sensor. In other words, while a print is being made by using a specific ink color more than any other ink color, the adhered mists are accordingly dominated by the specific color. As a result, the adhered mists play a function of a filter for absorbing another specific color. This not only reduces the amount of light emitted from each of the light-emitting elements, but also distorts their spectral characteristics.
FIG. 8A is a diagram showing a result of comparison between a pre-printing spectral characteristic of a blue LED and a post-printing spectral characteristic of the blue LED, which was observed after a printing operation was carried out by using even amounts of all of the ink colors, while FIG. 8B shows a result of comparison between a pre-printing spectral characteristic of a blue LED and a post-printing spectral characteristic of the blue LED, which was observed after a printing operation was carried out by using a yellow ink color only. In each of FIGS. 8A and 8B, the solid line indicates the pre-printing spectral characteristic of a blue LED, while the dashed line indicates the post-printing spectral characteristic thereof. All the values of the spectral characteristics are normalized to be shown.
FIG. 8A shows a pre-printing spectral characteristic and a post-printing spectral characteristic which was observed after a printing operation was carried out by using even amounts of four colors (i.e. cyan, magenta, yellow and black). There is no difference between these characteristics. On the other hand, FIG. 8B shows the pre-printing spectral characteristic and a post-printing spectral characteristic which was observed after a printing operation was carried out by using only the yellow ink. It is learned from FIG. 8B that the post-printing characteristic is distorted at the longer wavelength side. This stems from a fact that the spectral transmission factor of the yellow ink against the blue LED spectral characteristic is different to a large extent between the shorter wavelength side and the longer wavelength side. Through close examination, the present inventors have found that particularly a combination of complementary colors, that is, the combination of the blue LED with the yellow ink as well as a combination of a green LED with a magenta color, tends to affect the spectral characteristic of its corresponding LED. It goes without saying that the spectral characteristic is not necessarily distorted by only the continued use of a single color ink. This phenomenon more or less takes place even in a case where a printing operation is carried out with several ink colors in combination. However, in the case where the multiple different ink colors are used as shown in FIG. 8A, the combination of the spectral transmission characteristics of the respective ink colors affects the spectral characteristics of the LEDs. As a result, the combination of the spectral transmission characteristics varies the spectral characteristics thereof less than the spectral transmission characteristic of the single color ink does.
When the densities are measured by use of the LEDs with their own distorted spectral characteristics as shown in FIG. 8B, the reflected light and the result of the detection are adversely affected. This makes it impossible to obtain a correct result of the detection. This means that the repeatability of the density measurement is likely to deteriorate in a case where the same printing apparatus is used for a long time period. In addition, the sensor may cease to function normally even in a case where the same printing apparatus is used for a short time period, as long as a printing operation is carried out by using a specific ink color only.
Even though, as disclosed in Japanese Patent Application laid-open No. 2004-291601, the method of determining whether or not the optical sensor is usable on the basis of the value representing the output (or power) from each of the light-receiving sensors is adopted under this kind of condition, it is impossible to evaluate how the performance decreases due to the distorted spectral characteristics. If a user is frequently requested to provide a maintenance service to the sensor for assurance, the frequent request reduces the usability.